This invention relates to compositions of matter and methods of using them to remove selenium from fluids. Selenium compounds are reported to comprise 0.9 ppm of the earth's crust. Selenium is an important as a trace mineral used to make the enzyme glutathione peroxidase, which is involved in fat metabolism and therefore is found in many living organisms. It is commonly found in various amounts in crude oil, coal, and other fossil fuels originating from the decomposed organic matter or leached out of the nearby minerals. Selenium compounds are also found naturally in ground waters and in agricultural runoffs from the use of selenium containing insecticides and herbicides.
Unfortunately, selenium is known to be highly toxic and it can cause harm even in small quantities. Harmful effects include dermatitis, central nervous system disturbance, nephrosis, hemorrhagic necrosis of the pancreas and adrenal cortex, and when in large enough dosages, death. As a result, many localities have limited the permissible amount of selenium in domestic supplies of water at 10 ppb. As a result, wastewater produced from activity involving selenium-containing materials is difficult to dispose of. In addition, because of its toxicity, even these strict standards may be uniformly further limited in the future.
The chemical properties of selenium however make its removal from solutions difficult and complex. Although insoluble when in its elemental state, selenium has four oxidation states (−2, +2, +4, and +6), which allows it to readily form a number of compounds that are highly soluble and therefore very hard to remove from solution. (See Kapoor et al., Removal of Selenium from Water and Wastewater, Environmental Studies, Vol. 49, pp. 137-147 (1995)). As a result, prior art removal methods have been either disappointing or in some cases mostly ineffective. One prior art method, described in U.S. Pat. No. 7,419,602 involves the use of a ferric salt, pH adjustment, and an oxidant but in practice is less than 70% effective. Another method described in U.S. Pat. No. 5,510,040 describes a method using poly dithiocarbamate materials which while more effective also involves considerable expense.
Thus there is a clear need for and utility in an improved method of removing selenium from solution. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists.